My father, the Auschwitz commandant

Barbara Cherish's father was an SS officer who ran the concentration camp at Auschwitz. Adopted after the second world war by an American family, she kept her birth father's identity secret for decades. She tells Joanna Moorhead what it's like to be the child of a man responsible for mass murder, and how she finally faced the truth

At the age of 47, Barbara Cherish was at a crossroads. Her children were grown up, but her marriage was over. Her beloved elder sister, who had been a surrogate mother when she was young, had recently died. "Everything had gone from under me," she says. "But I knew one thing: this was the moment to confront the secret I had kept hidden for so long."

That secret lay buried in an upstairs drawer. It was a photograph of a man whose eyes Barbara describes as "thoughtful and sad", a man whose existence she had never dared to admit to anyone. The man was her father: the photograph betrayed the awful truth about him. In the picture, Arthur Liebehenschel is wearing a military hat - a hat that clearly displays the insignia of Hitler's SS and the death's head of the concentration camp staff.

Barbara's father wasn't just any SS officer - he had one of the most terrible jobs imaginable. "He was the camp commandant at Auschwitz," says Barbara quietly. "And, do you know, when I started being able to say that out loud, it was actually a tremendous relief."

"My father had an appalling job," says Barbara, a 66-year-old grandmother who lives in San Diego, California. "But I couldn't go on hiding it. I had to find out who he was and what he was, as much to find out where I came from as for any other reason." She says she wanted to uncover the truth as much for her son and her granddaughter as for herself (her daughter died several years ago). "I don't want there to be secrets in our family any more. However bad the truth, I wanted it to be known."

Barbara, named Bärbel when she was born in Nazi Germany in 1943, began her quest at that crossroads in 1988 after her sister's death. It was the start of a journey that has taken more than 20 years. The final element in the process is the publication of a book detailing all the information she has been able to muster about Liebehenschel and his life.

It's a book that is both unbearably moving and extraordinarily laborious - there are pages and pages of transcripts from his trial after the war ended. And yet, says Barbara, this was essential to her mission. "I wanted to let him speak in his own words," she says. "I wanted him to have a chance to say himself what his life was about."

Barbara is adamant that she wasn't seeking to exonerate a man who had played so central a role in the Final Solution. Yet it's clear, from the earliest pages, that she can't stop herself from being Liebehenschel's daughter. If there is good to see in the man she will - unsurprisingly - find it. So we are told that when he was put in charge of Auschwitz he released prisoners from the notorious Block II, and put an end to the executions at the Black Wall. Auschwitz survivors, some of whom Barbara has approached and interviewed, have told her that her father, while culpable for his part in the Nazi atrocities, did what he could to help some of the prisoners. And Barbara quotes Anneliese, Liebehenschel's lover, who remembers how he would return home from the camp and cry, before taking long showers and longer walks.

None of it, of course, adds up to any kind of pardon - and Barbara is well aware that there is likely to be criticism about what she has done. What she wants, she admits, is to provide, if not excuses, then a context to what her father did. She talks about him as a man who loved the military life, who was a career soldier, who perhaps wanted to extricate himself from the horror but didn't know how. To many readers this will ring hollow - when she dares to compare him to Oscar Schindler, who saved thousands of Jews from death, one feels she is sailing very close to the wind.

What is fascinating about Barbara is that the man she has managed to see in as favourable a light as possible was not only a war criminal (he was found guilty of war crimes and hanged in 1948) but also deserted her mother, leaving her to bring up four children alone. It was this desertion, in fact, that led to Gertrud Liebehenschel's mental breakdown, as a result of which Barbara was eventually adopted and shipped to the US with her new family.

So the man she is trying so hard to love is responsible not only for unquantifiable public suffering, but also for immense private grief. Yet, almost seven decades on, Barbara is moved to tears when she talks about him.

One is also struck, reading her account of what it was to be the child of a Nazi in the aftermath of Germany's defeat, that this is a story that has rarely, if ever, been told.

She remembers the family suitcases being stolen one day at a railway station, so that she and her sisters and mother were left with "no money ... just the clothes on our backs ... " and that the following winter was so cold that Brigitte, her eldest sister, got the beginnings of tuberculosis. At one point, the family was in such need that she and a little friend sat begging in the street.

But it's the earlier stories that are most fascinating. Her other sister, Antje, recalls, for example, prisoners from Sachsenhausen concentration camp - her father worked there before Auschwitz - coming to the house to build a conservatory. One of the prisoners used to give Antje rides on his shoulders, promising to build her a playhouse one day. When Bärbel herself was born, in 1943, her mother was awarded the bronze Mutterkreuz - the Mother's Cross - a medal awarded by Hitler to women who had four or more children (those who bore six received a silver medal, and those who bore eight got gold).

But, she adds: "At the precise moment of my birth, people were being herded into gas chambers, their babies never even given a chance to live." Later, when her father had left and moved to Auschwitz with his new lover (he was transferred as a punishment for getting divorced), she quotes a chilling entry from his journal, noting the birth of another son, Hans-Dieter, in the camp hospital: the child was, he says, "a special gift from God, a new life in the midst of where so many had to die". Barbara explains that the other women giving birth that day were all camp prisoners; their babies would be taken from them immediately and never seen again.

The story of Gertrud, Barbara's mother, is also tragic: having lost her husband to another woman, she was then derided after the war as the wife of an SS officer. Her descent into mental instability is charted painstakingly, with the young Bärbel trying to understand why her mother is taking her and Antje on a two-week walk across fields and through forests from their home in Germany to their former holiday house in Austria. Eventually, Brigitte - by then 16 - tracked them down, and rescued the starving Bärbel. Brigitte and Bärbel were fostered.

In fact, Barbara never saw her mother again, and Gertrud died in 1966 in a mental institution after being diagnosed with paranoid schizophrenia.

A couple of years after being rescued by Brigitte, Barbara was adopted by the sister and American brother-in-law of Brigitte's husband, giving Barbara a new father who had fought for the Allies against Hitler.

Leaving her family behind was very hard - but travelling by ship to the US and starting a new life there was hugely exciting. "Every day was a new adventure for me - I remember arriving in New York and driving upstate and how my eyes followed the road with such curiosity, imagining a whole new life unravelling around every corner. I missed my old family, of course, but having a fresh start was tremendous."

Her American family moved first to New York and then California, but Barbara's new mother warned her never to mention her father's past to anyone in America - and for three decades, she says, "I didn't tell a soul."

For Arthur Liebehenschel, too, the story ends with silence: by the time he was hanged for war crimes in 1948 he had been in prison for three years, and died without knowing what had become of the family he had abandoned.

At his trial, Liebehenschel denied knowledge of what was going on at Auschwitz, which, Barbara admits, is unbelievable. "He lied when he was being interrogated - that's pretty clear," she says. "Why he lied and how much he lied about, we can only speculate. But there were times when he seems to have gone to Berlin to try to stop the transports ... the truth is that the picture is complicated, but it's clear that he knew more about what was going on than he claimed at his trial."

It must be hard to feel sorrow for a war criminal who surely had forfeited the rights he took from so many other human beings, but decades on, and from across the world, his youngest daughter is still sad that her father's last days must have been lonely. "He didn't know where we were or what had become of us, or even whether we were still alive. In his final letter to his wife, he says how peculiar it is that there has been no news or trace of his little girls, and I find that so tragic."

Barbara's search has brought many tears and much sadness, but there is contentment, too, for her these days. Although she kept the truth a secret, she feels that every aspect of her life for all those years was lived under the cloud of her father's Nazi past. Coming out of the shadows, and naming that history, has brought her closure.

She feels that, whatever he did, she has done her best by him. Once, she says, she had a dream in which two hands extended down from the heavens towards her: but they were covered with long, black gloves, and she was terrified as she realised they were reaching for her. The moment the hands touched hers, though, she felt what she describes as "an overwhelming sense of love and peace". Liebehenschel may have been a monster, but he was also a father - and his daughter has never forgotten him.

• The Auschwitz Kommandant: A Daughter's Search for the Father She Never Knew by Barbara U Cherish is published by The History Press, £20. To order a copy for £18, with free UK p&p, go to theguardian.com/bookshop or call 0330 333 6846